Electrophotographic marking is a well-known, commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a charged photoreceptor with a light image representation of a desired document. The photoreceptor is discharged in response to that light image, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image, forming a toner image, which is then transferred onto a substrate, such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a permanent record of the original representation. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of other images.
One way of exposing the photoreceptor is to use a Raster Output Scanner (ROS). A ROS is typically comprised of a laser source (or sources), a pre-polygon optical system, a rotating polygon having a plurality of mirrored facets, and a post-polygon optical system. In a simplified description of operation light from the laser source is collimated and focused onto the rotating polygon by the pre-polygon optics; the light beam is reflected by the polygon's mirrored facets, and the reflected light is then directed by the post-polygon optical system into a finely focused light spot on the photoreceptor's surface. As the polygon rotates, the spot traces a path on the photoreceptor surface referred to as a scan line. By moving the photoreceptor as the polygon rotates the spot raster scans the surface of the photoreceptor. By modulating the laser beam with image information a predetermined latent image is produced on the photoreceptor.
Polygons typically are rotated at thousands, possible tens of thousands, of revolutions per minute. To achieve such rotational velocities polygons are usually mounted on air bearings. While several types of air bearing are known in the art, one type uses a housing with a centered, cantilevered stationary shaft and a rotating sleeve on the shaft. A multifaceted polygon is mounted on the rotating sleeve and the rotating sleeve forms part of a motor. Such a group of components is referred to as a motor polygon assembly (MPA). To create the air bearing, grooves (or flats) for generating air pressure are formed either around the outer surface of the stationary shaft or around the inner surface of the rotating sleeve. As the rotating sleeve turns air pressure is created between the shaft and the sleeve. That air pressure provides a low friction bearing that centers the rotating sleeve on the stationary shaft.
While motor polygon assemblies are beneficial, they are subject to various rotational defects. For example, wobble and jitter introduce scan line anomalies. While the post-polygon optical system is designed to reduce wobble and jitter, they are not totally effective. Additionally, the ROS optical components and their mounts have mechanical resonant frequencies that can be driven by a rotating motor polygon assembly. A prior art method of reducing rotational defects is to stiffen the housing and the cantilevered stationary shaft. While useful, this approach becomes increasingly more costly and bulky as stiffness is increased. Therefore, a new approach to reducing rotational defects in motor polygon assemblies having cantilevered stationary shafts would be beneficial.